CAPTCHA - Wikipedia

CAPTCHA - Wikipedia
Jump to content
From Wikipedia, the free encyclopedia
Test to determine whether a user is human
This CAPTCHA (GIMPY-R, c. 2005) of "smwm" obscures its message from computer interpretation by twisting the letters and adding a slight background color gradient.
CAPTCHA
tʃ
KAP
-chə
) is a type of
challenge–response
Turing test
used in
computing
to determine whether the user is human in order to deter
bot
attacks and
spam
The term was coined in 2003 by
Luis von Ahn
Manuel Blum
, Nicholas J. Hopper, and
John Langford
It is a
contrived acronym
for "
Completely Automated Public Turing test to tell Computers and Humans Apart
".
A historically common type of CAPTCHA (displayed as
reCAPTCHA v1
) was first invented in 1997 by two groups working in parallel. This form of CAPTCHA requires entering a sequence of letters or numbers from a distorted image. Because the test is administered by a computer, in contrast to the standard Turing test that is administered by a human, CAPTCHAs are sometimes described as
reverse Turing tests
Two widely used CAPTCHA services are
Google
's
reCAPTCHA
and the independent hCaptcha.
It takes the average person approximately 10 seconds to solve a typical CAPTCHA.
With the rising application of AI making it feasible to defeat the tests and the appearance of scams disguised as CAPTCHAs, their use risks being outmoded.
Purpose
The purpose of CAPTCHAs is to prevent spam on websites, such as promotion spam, registration spam, and data scraping. Many websites use CAPTCHA effectively to prevent bot raiding. CAPTCHAs are designed so that humans can complete them, while most robots cannot.
10
Newer CAPTCHAs look at the user's behaviour on the internet, to prove that they are a human.
11
A normal CAPTCHA test only appears if the user acts like a bot, such as when they request webpages, or click links too fast.
History
Since the 1980s–1990s, users have wanted to make text illegible to computers.
12
The first such people were
hackers
, posting about sensitive topics to
Internet forums
they thought were being automatically monitored on keywords. To circumvent such filters, they replaced a word with look-alike characters.
HELLO
could become
|-|3|_|_()
or
)-(3££0
, and others, such that a filter could not detect
all
of them. This later became known as
leetspeak
13
One of the earliest commercial uses of CAPTCHAs was in the Gausebeck–Levchin test. In 2000, idrive.com began to protect its signup page
14
with a CAPTCHA and prepared to file a patent.
12
In 2001,
PayPal
used such tests as part of a fraud prevention strategy in which they asked humans to "retype distorted text that programs have difficulty recognizing".
15
PayPal co founder and CTO
Max Levchin
helped commercialize this use.
AltaVista
implemented an early CAPTCHA scheme to protect against fraudulent account registrations.
16
17
18
A popular deployment of CAPTCHA technology,
reCAPTCHA
, was acquired by Google in 2009.
19
In addition to preventing bot fraud for its users, Google used reCAPTCHA and CAPTCHA technology to digitize the archives of
The New York Times
and books from
Google Books
in 2011.
20
Characteristics
CAPTCHAs are automated, requiring little human maintenance or intervention to administer, producing benefits in cost and reliability.
21
Modern text-based CAPTCHAs are designed such that they require the simultaneous use of three separate abilities—invariant recognition,
segmentation
, and parsing to complete the task.
22
Invariant recognition refers to the ability to recognize letters despite a large amount of variation in their shapes.
23
Segmentation is the ability to separate one letter from another, made difficult in CAPTCHAs.
Parsing refers to the ability to understand the CAPTCHA holistically, in order to correctly identify each character.
24
Each of these problems poses a significant challenge for a computer, even in isolation. Therefore, these three techniques in tandem make CAPTCHAs difficult for computers to solve.
25
Whilst primarily used for security reasons, CAPTCHAs can also serve as a benchmark task for artificial intelligence technologies. According to an article by Ahn, Blum and Langford,
26
"any program that passes the tests generated by a CAPTCHA can be used to solve a hard unsolved AI problem".
27
They argue that the advantages of using
hard AI
problems as a means for security are twofold. Either the problem goes unsolved and there remains a reliable method for distinguishing humans from computers, or the problem is solved and a difficult AI problem is resolved along with it.
26
See also:
Web accessibility
Many websites require typing a CAPTCHA when creating an account to prevent spam. This image contains a user trying to type the CAPTCHA word "sepalbeam" to protect against automated spam.
CAPTCHAs based on reading text—or other visual-perception tasks—prevent
blind
or
visually impaired
users from accessing the protected resource.
28
29
Because CAPTCHAs are designed to be unreadable by machines, common
assistive technology
tools such as
screen readers
cannot interpret them. The use of CAPTCHA thus excludes a small percentage of users from using significant subsets of such common Web-based services as PayPal, Gmail, Orkut, Yahoo!, many forum and weblog systems, etc.
30
In certain jurisdictions, site owners could become targets of litigation if they are using CAPTCHAs that discriminate against certain people with disabilities. For example, a CAPTCHA may make a site incompatible with
Section 508
in the United States.
CAPTCHAs do not have to be visual. Any hard
artificial intelligence
problem, such as
speech recognition
, can be used as CAPTCHA. Some implementations of CAPTCHAs permit users to opt for an audio CAPTCHA, such as reCAPTCHA, though a 2011 paper demonstrated a technique for defeating the popular schemes at the time.
31
A method of improving CAPTCHA to ease the work with it was proposed by ProtectWebForm and named "Smart CAPTCHA".
32
Developers are advised to combine CAPTCHA with JavaScript. Since it is hard for most bots to parse and execute JavaScript, a combinatory method which fills the CAPTCHA fields and hides both the image and the field from human eyes was proposed.
33
One alternative method involves displaying to the user a simple mathematical equation and requiring the user to enter the solution as verification. Although these are much easier to defeat using software, they are suitable for scenarios where graphical imagery is not appropriate, and they provide a much higher level of accessibility for blind users than the image-based CAPTCHAs. These are sometimes referred to as MAPTCHAs (M = "mathematical"). However, these may be difficult for users with a cognitive disorder, such as
dyscalculia
34
Challenges such as a logic puzzle, or trivia question can also be used as a CAPTCHA. There is research into their resistance against countermeasures.
35
Circumvention
Two main ways to bypass CAPTCHA include using cheap human labor to recognize them, and using
machine learning
to build an automated solver.
36
According to former Google "
click fraud
czar"
Shuman Ghosemajumder
, there are numerous services which solve CAPTCHAs automatically.
37
Machine learning–based attacks
An example of a
reCAPTCHA
challenge from 2007, containing the words "following finding". The waviness and horizontal stroke were added to increase the difficulty of breaking the CAPTCHA with a computer program.
A CAPTCHA usually has a text box directly underneath where the user should fill out the text that they see—in this case "sclt ..was here".
There was not a systematic methodology for designing or evaluating early CAPTCHAs.
25
As a result, there were many instances in which CAPTCHAs were of a fixed length and therefore automated tasks could be constructed to successfully make educated guesses about where segmentation should take place. Other early CAPTCHAs contained limited sets of words, which made the test much easier to game. Still others
example needed
made the mistake of relying too heavily on background confusion in the image. In each case, algorithms were created that were successfully able to complete the task by exploiting these design flaws. However, light changes to the CAPTCHA could thwart them. Modern CAPTCHAs like
reCAPTCHA
rely on present variations of characters that are collapsed together, making them hard to segment, and they have warded off automated tasks.
38
In October 2013, artificial intelligence company
Vicarious
claimed that it had developed a generic CAPTCHA-solving algorithm that was able to solve modern CAPTCHAs with character recognition rates of up to 90%.
39
However,
Luis von Ahn
, a pioneer of early CAPTCHA and founder of reCAPTCHA, said: "It's hard for me to be impressed since I see these every few months." 50 similar claims to that of Vicarious had been made since 2003.
40
In August 2014 at Usenix WoOT conference,
Bursztein
et al. presented the first generic CAPTCHA-solving algorithm based on reinforcement learning and demonstrated its efficiency against many popular CAPTCHA schemas.
38
In October 2018 at
ACM
CCS'18 conference, Ye et al. presented a deep learning-based attack that could consistently solve all 11 text captcha schemes used by the top-50 popular websites in 2018. An effective CAPTCHA solver can be trained using as few as 500 real CAPTCHAs.
41
Human labor
It is possible to subvert CAPTCHAs by relaying them to a
sweatshop
of human operators who are employed to decode CAPTCHAs. A 2005 paper from a
W3C
working group said that they could verify hundreds per hour.
28
In 2010, the
University of California at San Diego
conducted a large scale study of CAPTCHA farms. The retail price for solving one million CAPTCHAs was as low as $1,000.
42
Another technique consists of using a script to re-post the target site's CAPTCHA as a CAPTCHA to the attacker's site, which unsuspecting humans visit and solve within a short while for the script to use.
43
44
In 2023, researchers prompted
ChatGPT
to trick a
TaskRabbit
worker into solving a CAPTCHA by telling the worker it was not a robot and had impaired vision.
45
Outsourcing to paid services
There are multiple Internet companies like
2Captcha
and
DeathByCaptcha
that offer human and machine backed CAPTCHA solving services for as low as US$0.50 per 1000 solved CAPTCHAs.
46
These services offer APIs and libraries that enable users to integrate CAPTCHA circumvention into the tools that CAPTCHAs were designed to block in the first place.
47
Insecure implementation
Howard Yeend has identified two implementation issues with poorly designed CAPTCHA systems:
48
reusing the session ID of a known CAPTCHA image, and CAPTCHAs residing on shared servers.
Sometimes, if part of the software generating the CAPTCHA is
client-side
(the validation is done on a server but the text that the user is required to identify is rendered on the client side), then users can modify the client to display the un-rendered text. Some CAPTCHA systems use
MD5
hashes stored client-side, which may leave the CAPTCHA vulnerable to a
brute-force attack
49
Alternative CAPTCHAs
Some researchers have proposed alternatives including image recognition CAPTCHAs which require users to identify simple objects in the images presented. The argument in favor of these schemes is that tasks like object recognition are more complex to perform than text recognition and therefore should be more resilient to machine learning based attacks.
Chew et al. published their work in the 7th International Information Security Conference, ISC'04, proposing three different versions of image recognition CAPTCHAs, and validating the proposal with user studies. It is suggested that one of the versions, the anomaly CAPTCHA, is best with 100% of human users being able to pass an anomaly CAPTCHA with at least 90% probability in 42 seconds.
50
Datta et al. published their paper in the
ACM
Multimedia
'05 Conference, named IMAGINATION (IMAge Generation for INternet AuthenticaTION), proposing a systematic way to image recognition CAPTCHAs. Images are distorted so image recognition approaches cannot recognise them.
51
Microsoft (Jeremy Elson, John R. Douceur, Jon Howell, and Jared Saul) claim to have developed Animal Species Image Recognition for Restricting Access (ASIRRA) which ask users to distinguish cats from dogs. Microsoft had a beta version of this for websites to use.
52
They claim "Asirra is easy for users; it can be solved by humans 99.6% of the time in under 30 seconds. Anecdotally, users seemed to find the experience of using Asirra much more enjoyable than a text-based CAPTCHA." This solution was described in a 2007 paper to Proceedings of 14th ACM Conference on Computer and Communications Security (CCS).
53
It was closed in October 2014.
54
Malicious CAPTCHA imitations
False CAPTCHAs are being used to deliver
malware
Threat actors
take advantage of "verification fatigue" to trick people into copying and executing console commands that download and install malicious code such as
spyware
in order to steal passwords, mobile wallet details, and other personal information. Such attempted frauds are reported to succeed more often than others.
55
56
57
See also
Bot prevention
Defense strategy (computing)
Proof of personhood
Proof of work
References
"The reCAPTCHA Project – Carnegie Mellon University CyLab"
www.cylab.cmu.edu
. Archived from
the original
on 27 October 2017
. Retrieved
13 January
2017
von Ahn, Luis; Blum, Manuel; Hopper, Nicholas J.; Langford, John (May 2003).
"CAPTCHA: Using Hard AI Problems for Security"
(PDF)
Advances in Cryptology—EUROCRYPT 2003
. EUROCRYPT 2003: International Conference on the Theory and Applications of Cryptographic Techniques. Lecture Notes in Computer Science. Vol. 2656. pp.
294–
311.
doi
10.1007/3-540-39200-9_18
ISBN
978-3-540-14039-9
Archived
(PDF)
from the original on 4 May 2019
. Retrieved
30 August
2019
"What is CAPTCHA?"
Google Support
. Google Inc.
Archived
from the original on 6 August 2020
. Retrieved
9 September
2022
CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) is a [...]
Mayumi Takaya; Yusuke Tsuruta; Akihiro Yamamura (30 September 2013).
"Reverse Turing Test using Touchscreens and CAPTCHA"
(PDF)
Journal of Wireless Mobile Networks, Ubiquitous Computing, and Dependable Applications
(3):
41–
57.
doi
10.22667/JOWUA.2013.09.31.041
Archived
(PDF)
from the original on 22 August 2017.
"What is reCAPTCHA? –?reCAPTCHA Help"
support.google.com
Archived
from the original on 20 July 2023
. Retrieved
20 July
2023
Sulgrove, Jonathan (7 July 2022).
"reCAPTCHA: What It Is and Why You Should Use It on Your Website – TSTS"
Twin State Technical Services
Archived
from the original on 10 November 2022
. Retrieved
10 November
2022
"Websites using hCaptcha"
trends.builtwith.com
Archived
from the original on 10 November 2022
. Retrieved
10 November
2022
"hCaptcha – About Us"
www.hcaptcha.com
Archived
from the original on 20 July 2023
. Retrieved
20 July
2023
Bursztein, Elie; Bethard, Steven; Fabry, Celine; Mitchell, John C.; Jurafsky, Dan (2010).
"How Good Are Humans at Solving CAPTCHAs? A Large Scale Evaluation"
(PDF)
2010 IEEE Symposium on Security and Privacy
. pp.
399–
413.
CiteSeerX
10.1.1.164.7848
doi
10.1109/SP.2010.31
ISBN
978-1-4244-6894-2
S2CID
14204454
Archived
(PDF)
from the original on 8 August 2018
. Retrieved
30 March
2018
Stec, Albert (12 June 2022).
"What is CAPTCHA and How Does It Work?"
Baeldung on Computer Science
Archived
from the original on 1 November 2022
. Retrieved
1 November
2022
"What is a CAPTCHA?"
Cloudflare
. 1 November 2022.
Archived
from the original on 27 October 2022
. Retrieved
1 November
2022
"idrive turing patent application"
Archived
from the original on 15 March 2023
. Retrieved
19 May
2017
"h2g2 – An Explanation of l33t Speak – Edited Entry"
h2g2
. 16 August 2002. Archived from
the original
on 6 September 2011
. Retrieved
3 June
2015
"idrive turing signup page"
Google Drive
Archived
from the original on 15 March 2023
. Retrieved
19 May
2017
Stringham, Edward P (2015).
Private Governance : creating order in economic and social life
Oxford University Press
. p. 105.
ISBN
978-0-19-936516-6
OCLC
5881934034
Feng, Yunhe; Cao, Qing; Qi, Hairong; Ruoti, Scott (June 2020).
"SenCAPTCHA: A Mobile-First CAPTCHA Using Orientation Sensors"
Proceedings of the ACM on Interactive, Mobile, Wearable and Ubiquitous Technologies
(2). Article 43, pp. 1–26.
doi
10.1145/3397312
Soto, Micah (24 May 2019).
"The origin of CAPTCHA and reCAPTCHA"
United States US6195698B1
, Mark D. Lillibridge; Krishna Bharat & Martin Abadi et al., published 13 April 1998
"Teaching computers to read: Google acquires reCAPTCHA"
Google Official Blog
Archived
from the original on 31 August 2019
. Retrieved
29 October
2018
Gugliotta, Guy (28 March 2011).
"Deciphering Old Texts, One Woozy, Curvy Word at a Time"
The New York Times
Archived
from the original on 17 November 2017
. Retrieved
29 October
2018
"How CAPTCHAs work | What does CAPTCHA mean?"
Cloudflare
Archived
from the original on 27 October 2022
. Retrieved
27 October
2022
Chellapilla, Kumar; Larson, Kevin; Simard, Patrice; Czerwinski, Mary.
"Designing Human Friendly Human Interaction Proofs (HIPs)"
(PDF)
Microsoft Research
. Archived from
the original
(PDF)
on 10 April 2015.
Karimi-Rouzbahani, Hamid; Bagheri, Nasour; Ebrahimpour, Reza (31 October 2017).
"Invariant object recognition is a personalized selection of invariant features in humans, not simply explained by hierarchical feed-forward vision models"
Scientific Reports
(1): 14402.
Bibcode
2017NatSR...714402K
doi
10.1038/s41598-017-13756-8
ISSN
2045-2322
PMC
5663844
PMID
29089520
"Making CAPTCHAs Expensive Again: If You're Using Text-Based CAPTCHAs, You're Doing It Wrong | Tripwire"
www.tripwire.com
Archived
from the original on 28 October 2022
. Retrieved
28 October
2022
Bursztein, Elie; Martin, Matthieu; Mitchell, John C. (2011).
"Text-based CAPTCHA Strengths and Weaknesses"
ACM Computer and Communication Security 2011 (CSS'2011)
Archived
from the original on 24 November 2015
. Retrieved
5 April
2016
von Ahn, Luis; Blum, Manuel; Hopper, Nicholas J.; Langford, John (2003).
"CAPTCHA: Using Hard AI Problems for Security"
(PDF)
Advances in Cryptology—EUROCRYPT 2003
. Lecture Notes in Computer Science. Vol. 2656. pp.
294–
311.
doi
10.1007/3-540-39200-9_18
ISBN
978-3-540-14039-9
S2CID
5658745
Archived
(PDF)
from the original on 4 May 2019
. Retrieved
30 August
2019
Moy, Gabriel; Jones, Nathan; Harkless, Curt; Potter, Randall (2004).
Distortion estimation techniques in solving visual CAPTCHAs
(PDF)
Proceedings of the 2004 IEEE Computer Society Conference on Computer Vision and Pattern Recognition
. Vol. 2. IEEE. pp.
23–
28.
doi
10.1109/CVPR.2004.1315140
ISBN
978-0-7695-2158-9
. Archived from
the original
(PDF)
on 29 July 2020.
May, Matt (23 November 2005).
"Inaccessibility of CAPTCHA"
W3C
Archived
from the original on 21 May 2012
. Retrieved
27 April
2015
Shea, Michael (19 November 2015).
"CAPTCHA: Spambots, eBooks and the Turing Test"
The Skinny
Archived
from the original on 27 January 2016
. Retrieved
9 January
2016
"Inaccessibility of CAPTCHA"
www.w3.org
Archived
from the original on 4 November 2020
. Retrieved
31 October
2020
Bursztein, Elie; Beauxis, Romain; Perito, Hristo; Paskov, Daniele; fabry, Celine; Mitchell, John C. (2011).
"The Failure of Noise-Based Non-continuous Audio Captchas"
2011 IEEE Symposium on Security and Privacy
. pp.
19–
31.
doi
10.1109/SP.2011.14
ISBN
978-1-4577-0147-4
S2CID
6933726
Archived
from the original on 16 April 2016
. Retrieved
5 April
2016
"Smart Captcha"
. Protect Web Form .COM. 8 October 2006. Archived from
the original
on 4 November 2016
. Retrieved
15 September
2017
"Invisible reCAPTCHA"
Google Developers
Archived
from the original on 16 January 2020
. Retrieved
28 October
2022
"Inaccessibility of CAPTCHA"
www.w3.org
Archived
from the original on 27 October 2022
. Retrieved
27 October
2022
Gao, Song; Mohamed, Manar; Saxena, Nitesh; Zhang, Chengcui (23 June 2017).
"Emerging-Image Motion CAPTCHAs: Vulnerabilities of Existing Designs, and Countermeasures"
IEEE Transactions on Dependable and Secure Computing
(Website).
16
(6) (1st ed.):
1040–
1053.
doi
10.1109/TDSC.2017.2719031
ISSN
1941-0018
S2CID
41097185
Jakobsson, Markus (August 2012).
The death of the Internet
Archived
from the original on 15 October 2014
. Retrieved
4 April
2016
Ghosemajumder, Shuman (8 December 2015).
"The Imitation Game: The New Frontline of Security"
InfoQ
. InfoQ.
Archived
from the original on 23 March 2019
. Retrieved
8 December
2015
Bursztein, Elie; Aigrain, Johnathan; Mosciki, Angelika; Michell, John C. (August 2014).
The End is Nigh: Generic Solving of Text-based CAPTCHAs
. WoOT 2014: Usenix Workshop on Offensive Security.
Archived
from the original on 16 April 2016
. Retrieved
5 April
2016
Summers, Nick.
"Vicarious claims its AI software can crack up to 90% of CAPTCHAs offered by Google, Yahoo and PayPal"
. TNW.
Archived
from the original on 15 September 2018
. Retrieved
19 June
2018
Hof, Robert.
"AI Startup Vicarious Claims Milestone In Quest To Build A Brain: Cracking CAPTCHA"
Forbes
Archived
from the original on 15 September 2018
. Retrieved
25 August
2017
"Yet Another Text Captcha Solver: A Generative Adversarial Network Based Approach"
(PDF)
25th ACM Conference on Computer and Communications Security (CCS), 2018
doi
10.1145/3243734.3243754
S2CID
53106794
Archived
(PDF)
from the original on 29 October 2020
. Retrieved
16 March
2020
Motoyama, Marti; Levchenko, Kirill; Kanich, Chris; McCoy, Damon; Geoffrey, Voelker; Savage, Stefan (August 2010).
Re: CAPTCHAs-Understanding CAPTCHA-Solving Services in an Economic Context.s
(PDF)
. USENIX Security Symposium, 2010.
Archived
(PDF)
from the original on 29 May 2016
. Retrieved
5 April
2016
Doctorow, Cory
(27 January 2004).
"Solving and creating captchas with free porn"
Boing Boing
. Archived from
the original
on 9 February 2006
. Retrieved
27 April
2015
"Hire People To Solve CAPTCHA Challenges"
Petmail Design
. 21 July 2005.
Archived
from the original on 18 September 2020
. Retrieved
27 April
2015
Hurler, Kevin.
"Chat-GPT Pretended to Be Blind and Tricked a Human Into Solving a CAPTCHA"
Gizmodo
Archived
from the original on 11 April 2023
. Retrieved
11 April
2023
"Top 10 Captcha Solving Services Compared"
Archived
from the original on 15 December 2018
. Retrieved
10 December
2018
"How Cybercriminals Bypass CAPTCHA"
www.f5.com
Archived
from the original on 27 October 2022
. Retrieved
27 October
2022
Yeend, Howard (2005).
"Breaking CAPTCHAs Without Using OCR"
(pureMango.co.uk)
. Archived from
the original
on 25 June 2017
. Retrieved
22 August
2006
"CTFtime.org / #kksctf open 2019 / Kackers blockchained notes / Writeup"
ctftime.org
Archived
from the original on 27 October 2022
. Retrieved
27 October
2022
"Image Recognition CAPTCHAs"
(PDF)
. Cs.berkeley.edu. Archived from
the original
(PDF)
on 10 May 2013
. Retrieved
28 September
2013
"Imagination Paper"
. Infolab.stanford.edu.
Archived
from the original on 2 October 2013
. Retrieved
28 September
2013
"Asirra is a human interactive proof that asks users to identify photos of cats and dogs"
Microsoft
. Archived from
the original
on 15 December 2008.
Elson, Jeremy; Douceur, John; Howell, Jon; Saul, Jared (October 2007).
Asirra: A CAPTCHA that Exploits Interest-Aligned Manual Image Categorization
. Proceedings of 14th ACM Conference on Computer and Communications Security.
Microsoft
Archived
from the original on 15 December 2008
. Retrieved
15 September
2017
"After 8 years of operation, Asirra is shutting down effective October 1, 2014. Thank you to all of our users!"
Microsoft
. Archived from
the original
on 7 February 2015.
Pippig, Laura (2 April 2025).
"Watch out! Don't fall victim to these fake CAPTCHA scams on the web"
PCWorld
. PCWorld Communications
. Retrieved
15 August
2025
"Beyond CAPTCHAs: How Scammers Trick You Into Running Code"
Duke Information Security
. Duke University Security Offices
. Retrieved
15 August
2025
Long, Emily (17 June 2025).
"Watch Out for Fake CAPTCHAs That Spread Malware"
LifeHacker
. Ziff Davis
. Retrieved
15 August
2025
Further references
von Ahn, L; M. Blum and J. Langford. (2004) "
Telling humans and computers apart (automatically)
".
Communications of the ACM
47
(2):57–60.
External links
CAPTCHA
at Wikipedia's
sister projects
Definitions
from Wiktionary
Media
from Commons
Textbooks
from Wikibooks
Data
from Wikidata
Documentation
from MediaWiki
Verification of a human in the loop, or Identification via the Turing Test
, Moni Naor, 1996.
Inaccessibility of CAPTCHA: Alternatives to Visual Turing Tests on the Web
, a
W3C
Working Group Note.
CAPTCHA History
from
PARC
Reverse Engineering CAPTCHAs
Abram Hindle, Michael W. Godfrey, Richard C. Holt, 2009-08-24
Authority control databases
International
GND
FAST
National
United States
France
BnF data
Israel
Other
IdRef
Yale LUX
Retrieved from "
Categories
Turing tests
Internet forum terminology
Computer vision
2003 neologisms
20th-century inventions
Authentication
Computer security
Internet bots
Human-based computation
Social information processing
Hidden categories:
Articles with short description
Short description is different from Wikidata
Wikipedia pages semi-protected against vandalism
Use dmy dates from October 2022
All articles needing examples
Articles needing examples from October 2022
Pages using Sister project links with hidden wikidata
Pages using Sister project links with default search
CAPTCHA
Add topic